a) Field of the Invention
The present invention relates to a semiconductor laser and its manufacture method, and more particularly to a distributed feedback semiconductor laser with a built-in diffraction grating for defining an oscillation wavelength, and its manufacture method.
b) Description of the Related Art
A light source excellent in a single wavelength performance is required by an arterial optical communications system which provides long distance and high capacity transmission. Material of an optical fiber has inevitable wavelength dispersion which changes a diffractive index or light transmission speed with a wavelength. If there is wavelength dispersion in a communication wavelength band, the waveform of a light pulse is distorted as the pulse is transmitted in an optical fiber. If a single chrominance performance of a laser beam is made high, the effects of wavelength dispersion are weak so that excellent transmission characteristics can be realized.
A distributed feedback semiconductor laser (DFB laser) defines its oscillation wavelength by a diffraction grating formed in the laser structure. A DFB laser is therefore excellent in the single wavelength performance. The typical structure of a DFB laser will be described with reference to FIGS. 5A and 5B and FIGS. 6A and 6B.
FIGS. 5A and 5B are a perspective view and a cross sectional view showing the structure of a refractive index coupling type DFB laser. On the surface of an n-type semiconductor substrate 51, an n-type clad layer 52 is formed which has a periodical step structure and a relatively low refractive index. On this n-type clad layer 52, an n-type guide layer 53 is formed which has a periodical step burying shape and a relatively high refractive index. The clad layer 52 and guide layer 53 having different refractive indices constitute a diffraction grating.
On the n-type guide layer 53, a layer 54 of a relatively low refractive index is formed. On this layer 54, a quantum well active layer 55 is formed. This active layer 55 is an alternate lamination of well layers W and barrier layers B. The well layer W has a composition for a relatively long wavelength and a relatively high refractive index. The barrier layer B has a composition for a relatively short wavelength and a relatively low refractive index. On the active layer 55, a p-type guide layer 56 is formed which has a relatively low refractive index. The lamination up to the p-type guide layer 56 has a striped mesa shape.
FIG. 5B is a cross sectional view showing the details of the structure near the n-type clad layer 52 and n-type guide layer 53. The n-type guide layer 53 has a refractive index higher than that of the n-type clad layer 52. The periodical step structure of the clad layer 52 and guide layer 53 therefore forms a periodical structure in terms of a refractive index.
The quantum well active layer 55 made of an alternate lamination of the well layers W and barrier layers B is an active layer for amplifying light. Light distributes, in the vertical direction, in the active layer 55 and also in the regions higher and lower than the active layer 55. The light components existing in the region lower than the active layer 55 are influenced by the periodical structure of the refractive index formed by the clad layer 52 and guide layer 53. Namely, the periodical structure of the guide layer 53 and clad layer 52 functions as a diffraction grating.
Reverting to FIG. 5A, a p-type burying layer 61 and an n-type burying layer 62 are formed burying the periphery of the striped mesa structure. These mesa structure and burying layers can be manufactured by forming a striped hard mask of SiO2 or the like on the p-type guide layer 56, then mesa-etching the subject layers, and thereafter selectively growing the burying layers on the exposed semiconductor surface. Thereafter, the hard mask is removed.
On the p-type guide layer 56 and n-type burying layer 62, a p-type clad layer 63 and a p+-type contact layer 64 are formed. Insulating layers 65 of SiO2 or the like are formed on both sides of a striped region of the p+-type contact layer 64. A p-side electrode 20 is formed on the contact layer 64 and insulating layers 65. In the striped region having no insulating layer 65, the p-side electrode 20 contacts the contact layer 64 to selectively inject current. The current distribution is confined also by the burying layers 61 and 62 and concentrated upon the mesa region. An n-side electrode 19 is formed on the substrate bottom surface.
This DFB laser oscillates at a wavelength near a Bragg wavelength determined by the period of the diffraction grating and has a high single chrominance performance.
A diffraction grating such as shown in FIG. 5B has thick regions and thin regions of the n-type guide layer 53. Two longitudinal modes exist depending upon which one of the thick and thin regions corresponds to the antinode of a standing wave. The DFB laser shown in FIGS. 5A and 5B does not oscillate precisely at the Bragg wavelength, but probabilistically oscillates either at a long or short wavelength side, or oscillates in the two modes at the same time.
A structure (λ/4 shift structure) has been proposed, the structure defining the oscillation mode by forming a ¼ wavelength phase shifter in the central area of a diffraction grating. In order to obtain a stable oscillation at a Bragg wavelength of a laser having this structure, it is necessary to avoid the effects of light reflected from end surfaces. If light reflected from an end face reaches the diffraction grating region, the phase of the diffraction grating is interfered with the phase of reflected light.
In order to remove reflected light, it is necessary to form a non-reflection (antireflection) film on opposite end surfaces. When the non-reflection film is formed on both end surfaces, light approximately same in amount outputs from both the end surfaces. Therefore, the light utilization factor of a light radiator utilizing light output from one end surface is approximately halved. If light returns from the external of the laser, an oscillation spectrum becomes unstable.
A gain coupling DFB laser (complex coupling DFB laser) has been proposed as a laser having a higher reliability of an oscillation spectrum than that of a refractive index coupling DFB laser.
FIG. 6A is a schematic diagram showing the outline structure of an active layer of a gain coupling DFB laser. On an n-type clad layer 72, barrier layers (B) 73 and well layers (W) 74 are alternately laminated to form a multiple quantum well structure 75 (having the barrier layers 73 as its uppermost and lowermost layers).
The barrier layers (B) 73 and well layers (W) 74 are periodically removed from the surface of the multiple quantum well structure 75 to an intermediate depth thereof. In the structure shown in FIG. 6A, the regions to the half depth of the fourth barrier layer (B) 73 as counted from the uppermost barrier layer are removed. A p-type guide layer 76 is formed covering this periodical structure. On this p-type guide layer 76, a p-type clad layer 77 is formed.
In this gain coupling DFB laser, the multiple quantum well structure itself is periodically removed to form a diffraction grating. Carriers are injected also laterally into the well layers (W) 74 constituting the diffraction grating. Therefore, a gain of injection current periodically changes along a resonator direction and a large gain coupling coefficient can be obtained.
In the gain coupling DFB laser, the position (antinode position) of a large amplitude of a light standing wave generated along the resonator direction is fixed approximately to the position having a large gain. Therefore, the effects of external return light are rare and the oscillation spectrum is stable. The effects of the phase of light reflected at an end surface are also rare. Therefore, disturbance of an oscillation spectrum is small even for an asymmetric resonator structure having a non-reflection film formed only on a front or output end surface and a high reflection film formed on the rear end surface.
The manufacture processes for an active layer of such a gain coupling DFB laser will be described with reference to FIGS. 6B to 6D.
As shown in FIG. 6B, a multiple quantum well structure 75 is formed by alternately laminating barrier layers (B) 73 having a wide band gap and well layers (W) 74 having a narrow band gap. On the surface of this multiple quantum well structure 75, a resist mask 80 having a periodical structure is formed.
As shown in FIG. 6C, by using the resist mask 80 as an etching mask, the multiple quantum well structure 75 is etched to an intermediate depth. This etching is stopped at an intermediate depth of the fourth barrier layer (B) 73. The resist mask 80 is thereafter removed.
As shown in FIG. 6D, a p-type guide layer 76 is grown burying the step regions of the multiple quantum well structure 75 partially etched to have the periodical structure. Thereafter, mesa etching, burying layer forming, clad layer forming and contact layer forming are performed. In this manner, the multiple quantum well structure having the periodical structure as shown in FIG. 6A is formed.
In the gain coupling DFB laser, the active layer itself having a high refractive index is etched. Therefore, a complex coupling diffraction grating is formed which has both refractive index modulation and gain modulation at the same time along the resonator direction. Such a complex coupling DFB laser oscillates at a long wavelength side of a Bragg wavelength.
In the complex coupling DFB laser, carriers are injected also laterally into the upper well layers W in the stepped regions. Therefore, a large gain coupling coefficient can be obtained and a refractive index coupling coefficient can be set to a smaller value than that of the structure with all well layers in the active layer being etched. The above-described advantages of the complex coupling DFB laser can be utilized greatly.
For the fundamental structure and performance analysis of a gain coupling DFB laser, for example, refer to “Global and Local Effects in Gain-Coupled Multiple-Quantum-Well DFB Lasers” by A. Champagne et. al. IEEE J. Quantum Electron., vol. 35, pp. 1390-1401, 1999.
A quantum well active layer such as shown in FIG. 6A is formed, for example, by alternately laminating well layers about 5 nm in thickness and barrier layers about 10 nm in thickness. In the structure shown in FIG. 6A, etching is stopped at an intermediate depth of the fourth barrier layer as counted from the uppermost layer.
It is not easy to stop etching precisely at an intermediate depth of a barrier layer having a thickness of about 10 nm. When a variation in intra-wafer or inter-wafer etch rate distributions is taken into consideration, the number of etched well layers in the same wafer or among different wafers may change. As the number of etched well layers changes, gain coupling and refractive index coupling components change so that the characteristics of lasers change from one laser to another laser.
If the periodical structure is formed by dry etching, crystal defects may be introduced by this etching. If the etching of a barrier layer is not stopped at a desired position but the left barrier layer becomes too thin or the underlying well is exposed or even partially etched, then crystal defects are introduced into the well layer. These crystal defects increase non-radiative recombination centers.